docs(typed-policies): a store read failure is 503 storage_unavailable

[saurabhjain1592]

Summary

A v11.0.0 platform answers a document store it cannot read with 503 storage_unavailable, not nothing_active (getaxonflow/axonflow-enterprise#4255, closed by getaxonflow/axonflow-enterprise#4266; platform/orchestrator/typed_authoring_route.go:979 at axonflow-enterprise main 8a1e2d312). This SDK's docs still said the platform answers nothing_active for a store read failure. This change says what a v11.0.0 platform does, in the places #265 lists, pins it with a unit test, and keeps one accurate line for a stack built before the change:

• active()'s docstring (axonflow/typed_policies.py, the same three lines, so the line-keyed falsey-clobber baseline does not move), the README's active() bullet and the CHANGELOG entry (in [Unreleased]): a 503 storage_unavailable from active() raises TypedPolicyRefusal, citing getaxonflow/axonflow-enterprise#4255.
• TestTheRefusals::test_every_documented_refusal_is_typed gains active-503-storage-unavailable: active()'s 503 storage_unavailable is a TypedPolicyRefusal carrying that status and reason.
• runtime-e2e/v11_examples/README.md: "And on a stack built before getaxonflow/axonflow-enterprise#4255, an unreadable document store also answers nothing_active."

active() already raised every answer other than 404 nothing_active as a typed refusal, so there is no code change: no behaviour change; nothing a stack could observe moved, and there is no live leg. The TypeScript SDK carries the same change (getaxonflow/axonflow-sdk-typescript#290), and so does the Java SDK (getaxonflow/axonflow-sdk-java#248).

Closes #265
Part of getaxonflow/axonflow-enterprise#3746

Local round

Every step at the head fd7f4fc, each under its own build-slot ticket, in a Python 3.11 virtual environment with the SDK installed editable from this tree (checked before the round), mirroring sdk-python's required contexts; before each step the round checks that the head is unchanged and the tree clean.

Step	Command	rc
lint	ruff check . and ruff format --check .	0
types	mypy axonflow (strict)	0
falsey-clobber	scripts/lint_no_falsey_clobber.py axonflow/ --baseline .lint_baselines/falsey_clobber.json (the docstring keeps its line count, so the line-keyed baseline does not move)	0
AuthZEN types	scripts/gen_authzen_types.py --check	0
test	pytest: 1616 passed, 30 skipped; coverage 85.80% against the configured 75%	0
no mocks	scripts/lint-no-mocks-in-runtime-e2e.sh	0
wire shape	pytest tests/test_wire_shape.py -m wire_shape against tests/fixtures/openapi	0
examples	py_compile and import of both examples	0

The new row ran by name: PASSED tests/test_typed_policies.py::TestTheRefusals::test_every_documented_refusal_is_typed[active-503-storage-unavailable].

CI notes

The board at the head is read by name once it completes; this section is filled in then.